1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling the pressure in a holding furnace incorporated in a low-pressure diecasting system. More specifically, the present invention relates to a method of and an apparatus for controlling by a microcomputer a proportional pressure control valve provided in a pressurized gas supply circuit for supplying gas under pressure to a holding furnace.
2. Prior Art
In order to change the pressure in a holding furnace of a low pressure die-casting apparatus in accordance with a preselected time--pressure characteristic curve (hereinafter referred to as a "desired pressurization pattern"), it has recently been proposed to provide a proportional pressure control valve in a circuit which supplies pressurized gas into the holding furnace, and to operate that proportional pressure control valve by means of a microcomputer. The proportional pressure control valve is constructed such that the secondary pressure is set when the opening of the valve's sleeve is closed by balancing the attraction force of a proportional solenoid and a force generated by the secondary pressure acting on the end surface of a spool via a feed-back path provided in the sleeve.
However, such proportional pressure control valves suffer from certain problems. That is, when the pressure of the holding furnace is increased to a predetermined desired pressure, such a control pattern inevitably arises wherein the pressure within the holding furnace oscillates about the desired pressure, having an amplitude which gradually decreases and finally converges to the desired pressure. Such a pressure oscillation causes the level of molten metal in the cavity of a mould to go up and down, resulting in defects in casting. It takes time to bring the valve into an operable condition which allows gas under a predetermined pressure to flow therethrough when any of various current values is sent as a command signal. In other words, there is a time lag between the input of the command signal and the occurrence of gas flow under a predetermined pressure. As a result, when the pressure in the holding furnace is to be controlled in accordance with a desired pressurization pattern by changing the pressure of the gas to be supplied to the furnace as time elapses, the pressure may not actually be changed on the basis of that pattern. Further, there is a limitation to the size of proportional pressure control valves, and this makes it impossible for a valve to deal with the demand for provision of a larger sized holding furnace which requires a larger amount of pressurized gas.